CMI Credit Management lp "Falsely attempting to collect a debt already settled with another agency on an account they say they can not locate without my social security number" Carrollton, Texas

I received a collection letter from Collections Unlimited Tx INC in October 2010 for a bad check that was supposedly written in October of 2003. Seven years is beyond the statute of limitations for being held responsible for a check, and the collection should have been dropped.

Upon months of arguing and requesting proof of such debt, I received a copy of the check. I wrote a new check to cover the debt and the account should have been closed. This check was written and sent on March 1, 2011 certified mail. They received the check the very next day and has still not cashed it.

Two days ago, I received a letter from CMI (credit management, lp) in Carrollton, Tx stating that they received my letter (which accompanied the check written and sent on 3/1/11) but could not process my request because they could not find the account refferenced in the letter. They proceded to ask for personal information such as my Social Security Number. This was the first I have heard from CMI during this entire ordeal, and they were not on the list of companies that I sent a copy of the letter to.

I do not know how they recieved my letter so I called them to find out what is going on. I spoke with Humberto who said that since there is no reference number on the bottom left corner of the letter, there is no account number and he was unable to find the account with the address or name in the letter, to disregard.

CMI may be a legitimate collection agency, however someone is sending letters with their name and address to collect personal information. My fear is that this mistake after mistake after mistake will hurt my credit, and all for nothing that I did wrong. What should be my next step at this point?